1. Field of the Invention
The present invention relates to a flat panel display and a method for driving the same, particularly to a flat panel display and a method for driving the same that enhance a characteristic of a thin film transistor by supplying a back bias to a conductive substrate through a metal member connected with an external system control panel and a ground unit formed on a substrate-exposing part of the conductive substrate, and/or that avoid cutdown and/or fluctuation of a threshold voltage generated by a lot of static and/or avoid undersired voltage from being supplied to the conductive substrate.
2. Discussion of Related Art
Recently, a technique for forming a thin film transistor by using a semiconductor layer formed on a substrate having an insulating surface has received a lot of attention. The thin film transistor can be broadly applied to an electronic device such as an Integrated Circuit (IC) or an electro optical apparatus. Since glass and/or quartz is used to form the thin film transistor, the thin film transistor can be fragile and/or relatively thick and heavy. However, an electronic device, particularly for example, a display device that uses the thin film transistor, should be light and not be too fragile.
In order to solve the problems associated with using a glass substrate, a material that is relatively thin and non-fragile has been suggested. For example, a thin film transistor formed on a conductive substrate has been suggested, and a switching element and a driver circuit of a flat display panel, which uses a conductive substrate material, are actively being studied.
Referring to FIGS. 1 and 2, a conventional flat display panel including a conductive substrate forming a thin film transistor is shown.
FIG. 1 illustrates a simplified plan view of a flat panel display 1′ according to the conventional art, and FIG. 2 illustrates a sectional view of a pad unit taken along the I-I′ line in FIG. 1. Referring to FIGS. 1 and 2, the flat panel display 1′ according to the conventional art includes a conductive substrate 10 forming an image display unit 11 and a pad unit 12, and an interface panel 14 electrically connected with the pad unit 12.
The picture display unit (or image display unit) 11 formed on the conductive substrate 10 includes a plurality of thin film transistors (not shown), and the pad unit 12 includes a buffer layer 22, an interinsulating layer 23 formed on the buffer layer 22, a plurality of terminals 13 formed on the interinsulating layer 23, and a lower insulating layer 20 formed on the under side of the conductive substrate 10.
The interface panel 14 forms a plurality of interface terminals 15, wherein the plurality of interface terminals 15 are formed on a position corresponding to the terminals 13 formed on the pad unit 12 and are to electrically connect with the terminals 13. Generally, the interface panel 14 can be divided into an embedded type storing a controller at its inside or an external type connected with a controller from the outside, and a Flexible Printed Circuit Board (FPCB) and an Integrated Circuit (IC) can be mainly used.
As mentioned above, one of characteristics of the conductive substrate 10, which forms a thin film transistor and/or the flat display panel 1′, is that a threshold voltage of the thin film transistor can be changed in accordance with a back bias voltage applied to the conductive substrate 10.
FIG. 3 illustrates electrical characteristic cutdown in accordance with a back bias voltage supplied to a thin film transistor.
As shown in FIG. 3, the X-axis represents a gate bias voltage supplied to a thin film transistor, and the Y-axis represents a drain current. As depicted in a graph, if a back bias voltage value supplied on a conductive substrate is high, a shift of a threshold voltage is increased. As a result, the threshold voltage can be tuned as a circuit type by adjusting a back bias voltage supplied on a conductive substrate forming a thin film transistor.
However, in a structure forming a thin film transistor on a conductive substrate, in the case that an undesired voltage is supplied to the conductive substrate caused by a lot of static and/or that an unpredicted electric charge is accumulated on the conductive substrate, a tuning as a circuit type becomes very difficult because a pre-adjusted threshold voltage of a thin film transistor is different from a reference designation value. Furthermore, it is problematic in that when a threshold voltage of a thin film transistor formed on a conductive substrate is different from a reference designation value, luminance, gray scale, and contrast that display respective colors displayed on a display device using a thin film transistor are changed.